1. Field of the Invention
This invention pertains to an improved process for removing copper and copper oxide-containing encrustations from ferrous metal surfaces. The invention is particularly applicable in the industrial cleaning of steam generating equipment which contains copper or copper alloys as component parts.
2. Description of the Prior Art
U.S. Pat. No. 3,072,502 (Alfano) describes a method of cleaning steam generating equipment with citric acid. This process is a two step process. The first step is conducted at an acidic pH to remove iron oxide scale and the second step of the process is conducted at an alkaline pH to remove copper. An oxidizing agent is said to be beneficial in the alkaline step. Air introduced by a sparger was illustrated as an embodiment.
U.S. Pat. No. 3,248,269 (Bell) described the use of neutral ammonium citrate at a high temperature and pressure to remove magnetic iron oxides and copper oxides from metal surfaces. The ammonium citrate thermally decomposes to form ammonia in situ. Air, oxygen, or other oxidizing agents were alleged to be helpful in the removal of copper. E.g. air introduced by way of a sparger.
U.S. Pat. No. 3,308,065 and U.S. Pat. No. Re. 30,796 (Lesinski) described a method of removing iron oxide scale from ferrous metal surfaces using aminated or ammoniated chelants at a pH of from about 8 to about 11. As an example of the chelants, Lesinski mentioned ethylenediaminetetraacetic acid (EDTA) and hydroxyethylethylenediaminetriacetic acid (HEDTA) Lesinski taught that the clean metal surface was passivated when the cleaning solution was drained away, exposed to air, and then flushed with fresh water.
U.S. Pat. No. 3,413,160 (Teumac) taught that ferrous metal surfaces cleaned with alkaline chelant solutions were passivated by introducing an oxidizing agent (e.g. air) into the solution while monitoring the oxidation potential. Passivation was said to occur when the oxidation potential was in the range of -250 to -175 millivolts (mV). Corrosion was said to occur if the oxidation step was carried too far. Accuracy and assurance of avoiding corrosion were increased by adding the oxidizing agent slowly enough for prompt reaction and prompt indication of the state of the solution, as shown by the oxidation potential. Column 3, line 69-73 and column 5, lines 60 et seq.
U.S. Pat. No. 3,438,811 and U.S. Pat. No. Re. 30,714 (Harriman) described a method of removing copper and copper oxide encrustations from a ferrous metal surface. The solvent used comprised an aqueous solution of a ferric chelant (e.g. ferric EDTA) at an alkaline pH. Harriman taught that as the copper dissolution proceeded, it was necessary to add an oxidizing agent to the cleaning solution to oxidize ferrous ion to ferric ion. Harriman taught that a variety of oxidizing agents could be used, including hydrogen peroxide, but that air was preferred. The examples illustrated this by showing air sparged into the system continuously during the copper removing process.
The Lesinski, Teumac and Harriman references represent the state of the art as far as industrial cleaning of steam generating equipment is concerned. A commercial process has been developed based upon these patents in which ammoniated EDTA salts at an alkaline pH have been used to clean iron oxide-containing scale from steam generating equipment, then if copper has been plated onto the freshly cleaned iron surface, it is removed by merely introducing an oxidizing agent (e.g. air) into the system in controlled amounts to thereby remove the copper. Thus, the same solvent is used throughout the entire procedure and the iron EDTA chelate is a solvent for the plated copper when air is introduced in controlled amounts to thereby produce ferric chelate. This cleaning technique also leaves the clean metal surface passive and it is an excellent cleaning process, but it can be somewhat extended from a time standpoint. For example, the "air blow down" can take several hours.